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United States Patent |
5,783,862
|
Deeney
|
July 21, 1998
|
Electrically conductive thermal interface
Abstract
A thermal interface 26 between a heat source (e.g., an IC die) 24 and a
heat sink 28 comprises a metallic mesh (26a) filled with a thermally
conductive semi-liquid substance (26b). The thermally conductive
semi-liquid substance may comprise, e.g., silicone grease or paraffin. The
wire mesh may comprise silver, copper and/or gold cloth.
Inventors:
|
Deeney; Jeffrey L. (Ft. Collins, CO)
|
Assignee:
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Hewlett-Packard Co. (Palo Alto, CA)
|
Appl. No.:
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938282 |
Filed:
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September 26, 1997 |
Current U.S. Class: |
257/714; 165/104.17; 165/185; 257/719; 257/720; 257/746; 257/747; 257/E23.112; 361/708; 361/712 |
Intern'l Class: |
H01L 023/34 |
Field of Search: |
165/104.17,185
257/714,719,720,746,747
361/705,707,708,712
|
References Cited
U.S. Patent Documents
3226608 | Dec., 1965 | Coffin, Jr. | 257/746.
|
3399332 | Aug., 1968 | Savolainen | 257/746.
|
3600787 | Aug., 1971 | Lindsay | 29/254.
|
3694699 | Sep., 1972 | Snyder et al. | 317/100.
|
3780356 | Dec., 1973 | Laing | 165/185.
|
3971435 | Jul., 1976 | Peck | 257/715.
|
4000509 | Dec., 1976 | Jarvela | 257/719.
|
4057101 | Nov., 1977 | Ruka et al. | 165/185.
|
4130233 | Dec., 1978 | Chisholm | 228/126.
|
4295151 | Oct., 1981 | Nyul et al. | 257/746.
|
4299715 | Nov., 1981 | Whitfield et al. | 165/185.
|
4384610 | May., 1983 | Cook et al. | 165/80.
|
4446916 | May., 1984 | Hayes | 165/104.
|
4482912 | Nov., 1984 | Chiba | 257/746.
|
4612978 | Sep., 1986 | Cutchaw | 257/714.
|
4639829 | Jan., 1987 | Ostergren et al. | 257/719.
|
4654754 | Mar., 1987 | Daszkowski | 361/388.
|
4915167 | Apr., 1990 | Altoz | 165/185.
|
5180001 | Jan., 1993 | Okada et al. | 165/185.
|
5323294 | Jun., 1994 | Layton et al. | 257/714.
|
5325913 | Jul., 1994 | Altoz | 165/32.
|
5402004 | Mar., 1995 | Ozmat | 257/718.
|
5404272 | Apr., 1995 | Lebailly et al. | 257/714.
|
5459352 | Oct., 1995 | Layton et al. | 257/718.
|
5561590 | Oct., 1996 | Norell et al. | 257/714.
|
5572404 | Nov., 1996 | Layton et al. | 257/714.
|
Other References
Hwang & Oktay, Thermal Interface Conduction Pad, IBM Tech. Dis. Bull. vol.
21 No. 10 Mar. 1979 p. 4028.
|
Primary Examiner: Ostrowsi; David
Attorney, Agent or Firm: Neudeck; Alexander J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION(S)
This application is a continuation of application Ser. No. 08/720,262 filed
on Sep. 26, 1996, now abandoned; which is a continuation of application
Ser. No. 08/514,349 filed on Aug. 4, 1995, now abandoned; which is a
continuation of application Ser. No. 08/201,131 filed on Feb. 24, 1994,
now abandoned; which is a continuation of application Ser. No. 07/855,370
filed on Mar. 20, 1992, now abandoned.
Claims
What is claimed is:
1. A thermal and electrical interface for conducting heat and electricity
which may be non-destructively assembled and disassembled, comprising:
a first member;
a second member; and
a woven mesh screen, said woven mesh screen being comprised of metal wires,
said metal wires being woven to form interstitial voids between said metal
wires, said interstitial voids being filled with a non-metallic thermally
conductive wetting compound which is not permanently solid, and said woven
mesh screen being compressed between said first member and said second
member, said metallic mesh screen also contacting said first member and
said second member to form a plurality of electrically and thermally
conductive paths between said first member and said second member through
said woven mesh screen.
2. A thermal and electrical interface according to claim 1, wherein said
non-metallic thermally conductive wetting compound comprises silicone
grease.
3. A thermal and electrical interface according to claim 2, wherein said
woven mesh screen is comprised of silver wire.
4. A thermal and electrical interface according to claim 2, wherein said
woven mesh screen is comprised of copper wire.
5. A thermal and electrical interface according to claim 2, wherein said
woven mesh screen is comprised of gold wire.
6. A thermal and electrical interface according to claim 1, wherein said
non-metallic thermally conductive wetting compound comprises paraffin.
7. A thermal and electrical interface according to claim 6, wherein said
metallic mesh screen is comprised of silver wire.
8. A thermal and electrical interface according to claim 6, wherein said
woven mesh screen is comprised of copper wire.
9. A thermal and electrical interface according to claim 6, wherein said
woven mesh screen is comprised of gold wire.
10. A thermal and electrical interface according to claim 1, wherein said
woven mesh screen is comprised of silver wire.
11. A thermal and electrical interface according to claim 1, wherein said
woven mesh screen is comprised of copper wire.
12. A thermal and electrical interface according to claim 1, wherein said
woven mesh screen is comprised of gold wire.
13. A thermal and electrical interface according to claim 1, further
comprising a protective plating on said metal wires.
14. A thermal and electrical interface for conducting heat and electricity
which may be non-destructively assembled and disassembled, comprising:
an integrated circuit;
a heat sink; and
a woven mesh screen said woven mesh screen being comprised of metal wires,
said metal wires being woven to form interstitial voids between said metal
wires, said interstitial voids being filled with a non-metallic thermally
conductive wetting compound which is not permanently solid, said woven
mesh screen contacting said integrated circuit and said heat sink to form
a plurality of electrically and thermally conductive paths between said
integrated circuit and said heat sink through said woven mesh screen, said
woven mesh screen also being compressed between said integrated circuit
and said heat sink by a compressive force which may be removed to
disassemble said interface and applied to assemble said interface.
15. A thermal and electrical interface according to claim 14, wherein said
non-metallic thermally conductive wetting compound comprises paraffin.
16. A thermal and electrical interface according to claim 14, wherein said
non-metallic thermally conductive wetting compound comprises silicone
grease.
17. A thermal and electrical interface according to claim 14, wherein said
woven mesh screen is comprised of gold wire.
18. A thermal and electrical interface according to claim 14, further
comprising a protective plating on said metal wires.
19. A thermal and electrical interface according to claim 14, wherein said
woven mesh screen is comprised of silver wire.
20. A thermal and electrical interface according to claim 14, wherein said
woven mesh screen is comprised of copper wire.
21. A thermal and electrical interface according to claim 14, wherein said
integrated circuit further includes a first surface and said heat sink
further includes a second surface, and said woven mesh screen comprises a
single layer disposed parallel to said first surface and said second
surface.
22. A method of thermally and electrically interfacing an integrated
circuit and a heat sink, comprising the step of:
compressing a woven mesh screen between said integrated circuit and said
heat sink, said woven mesh screen being comprised of metal wires, said
metal wires being woven to form interstitial voids between said metal
wires, said interstitial voids being filled with a non-metallic thermally
conductive wetting compound which is not permanently solid said woven mesh
screen also contacting said integrated circuit and said heat sink to form
a plurality of electrically and thermally conductive paths between said
integrated circuit and said heat sink through said woven mesh screen.
Description
FIELD OF THE INVENTION
The present invention generally relates to thermal interface materials, and
more particularly relates to an electrically conductive thermal interface
for coupling an integrated circuit (IC) die to a heat sink.
BACKGROUND OF THE INVENTION
A preferred application of the present invention is as an interface between
a high-density integrated circuit die and a metallic heat sink for
maintaining the temperature of the IC die below a prescribed maximum
temperature. Such an interface is also required in many other
applications, for example, between a power transistor and a heat sink or
between any other high power device packaged in a metallic case and a heat
sink.
As mentioned above, the object of heat sinking is to keep the operating
temperature of a relevant junction of the IC or other device below some
maximum specified temperature. For silicon transistors in metal packages,
for example, the maximum junction temperature of the IC is typically
200.degree. C., whereas for transistors in plastic packages it is usually
150.degree. C. The problem of designing an effective heat sinking system
can be stated as follows: Given the maximum power the device will
dissipate in a given circuit, one must calculate the relevant junction
temperature, allowing for the effects of thermal conductivity in the IC,
interface, heat sink, etc., and the maximum ambient temperature in which
the circuit is expected to operate. A heat sink large enough to keep the
junction temperature well below the maximum specified operating
temperature must then be chosen. To determine the overall thermal
performance of a packaging system, the designer must take into account the
thermal resistance (.theta..sub.c), of each component in the thermal path.
The thermal conductivity (K.sub.c), defined in units of Watts per meter
per degree Kelvin, is used to calculate the thermal resistance of each
element.
In many instances the heat sink is required to be electrically insulated
from the integrated circuit die, however there are also situations in
which the heat sink serves as a ground or source of back gate voltage,
e.g., in circuits for NMOS devices that require a fixed voltage on the
silicon. In cases where the heat sink serves as a source of ground or back
gate voltage, the interface between the heat sink and the IC die must be
electrically conductive as well as thermally conductive.
FIG. 1 depicts an example of an IC assembly that employs an electrically
and thermally conductive interface between a heat sink and an IC die. The
assembly of FIG. 1 comprises an extruded aluminum heat sink 10, an epoxy
preform 12, a heat spreader 14, a support ring 16, an encapsulated IC die
18, a tape-automated bonding (TAB) circuit 20 and a PC board 22. An
extruded aluminum heat sink is employed in the assembly of FIG. 1 because
of its ability to efficiently dissipate large amounts of thermal energy.
However, because of the high coefficient of thermal expansion (CTE) and
difficulty of making reliable electrical connections, the die 18 is not
directly attached to the aluminum heat sink 10. An intermediate heat
spreader 14 is placed between the die 18 and the heat sink 10. A b-stage
epoxy preform 12 is employed to attach the heat sink 10 to the heat
spreader 14, and an electrically conductive silver-filled thermoset epoxy
(not shown) is used to attach the die 18 to the heat spreader 14. When the
assembly is bolted together, knurled fasteners, pressed into two corners
of the heat spreader, provide back gate electrical connections to plated
pads on the backside of the PC board 22. The interface used to attach the
die to the heat spreader (silver-filled thermoset epoxy in the assembly of
FIG. 1) is required to have high thermal and electrical conductivity, high
resistance to moisture, and a degree of mechanical compliance.
A problem with known epoxy-based interface materials is that they prevent
the IC die from being detached from the heat sink after the epoxy has set.
This characteristic is a particular disadvantage in the design and
development stage, during which the assembly is typically assembled and
disassembled many times. Another problem with rigid epoxy-based interface
materials is that the difference in the CTE of the heat sink and the CTE
of the substrate can cause a large stress to be placed on the IC die when
the heat sink and substrate expand or contract. Accordingly, a primary
goal of the present invention is to provide an electrically conductive
thermal interface material that possesses high thermal and electrical
conductivity. Further goals of the invention are to provide an interface
that allows the assembly to be assembled and disassembled many times
without damaging the interface or other components of the assembly, and
that minimizes the stress placed on the die as a result of thermal
expansion of the heat sink and substrate.
SUMMARY OF THE INVENTION
The present invention provides an electrically conductive thermal interface
that achieves high thermal and electrical conductivity by means of a
pressure connection that can be assembled and disassembled many times. An
electrically conductive thermal interface in accordance with the present
invention comprises a metallic mesh filled with a thermally conductive
semi-liquid substance that exhibits viscous flow upon heating. The
thermally conductive semi-liquid substance may comprise, e.g., silicone
grease, paraffin or mineral-filled paraffin (e.g., paraffin filled with
alumina or aluminum nitride). The wire mesh may be made of any one of a
variety of metals, including silver, copper or gold. (Relevant
considerations are thermal conductivity, ductility and cost.) In addition,
there may also be a protective plating on the wire mesh to enhance the
electrical performance of the interface.
Examples of the invention have exhibited thermal conductivities comparable
to that of silver-filled epoxy (K.sub.c =1.9 W/m-.degree.K.). For example,
with a contact pressure of between 15 and 100 pounds per square inch,
K.sub.c was found to be 1.5 W/m-.degree.K. for a bare mesh, 2.3
W/m-.degree.K. for a paraffin-filled mesh and 2.7 W/m-.degree.K. for a
mesh filled with thermal grease. These conductivities will of course vary
in accordance with the pressure used to push the heat source toward the
heat sink (i.e., depending upon how tightly the fibers of the wire mesh
are packed).
Prior art thermal interfaces do not supply both the good thermal and
electrical conductivity required for some applications. By using a metal
mesh, good thermal conductivity is obtained when the fibers are pressed
together to effectively form hundreds of thermal "vias". Reliable
electrical connections are obtained at the high pressure areas where the
loops of the mesh contact the components. Unlike metal-filled epoxies, the
assembly can be assembled and disassembled many times without damaging the
IC. The electrical conductivity of the interface is similarly a function
of the pressure applied to the interface.
In addition, the present invention provides a compliant, stress-free
interface, which allows the heat sink to be directly attached (e.g.,
without a heat spreader) to a wide variety of substrates, such as copper
and aluminum. (In choosing a substrate, the electrical contact properties
of the surface must be considered; for example, aluminum forms hard,
non-conductive oxides.) Other features of the invention are described
below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts an exploded assembly view of a prior art integrated circuit
assembly.
FIG. 2 depicts a cross-sectional view of an apparatus comprising a heat
source coupled to a heat sink by an electrically conductive thermal
interface in accordance with the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 2 schematically depicts a preferred application of a thermal interface
in accordance with the present invention. This application includes a heat
source (e.g., an IC die) 24, an interface 26 and a heat sink 28. The
interface 26 comprises a metallic mesh, or clothe, 26a filled with a
thermally conductive semi-liquid substance 26b (i.e., the semi-liquid
substance 26b fills, at least partially, the interstices between the
respective wires of the mesh 26a). The thermally conductive semi-liquid
substance may comprise, e.g., silicone grease, paraffin or mineral-filled
paraffin.
The wire mesh may be made of any one of a variety of metals, including (but
not limited to) silver, copper and/or gold. In addition, the wire mesh may
advantageously be coated with a protective plating (not shown) to enhance
electrical performance. Plating with 30-50 microinches of nickel followed
by a gold flash (e.g., 5-10 microinches) will improve the electrical
conductivity of the interface; however, the hardness of nickel may
decrease the ductility of the mesh and thus reduce the thermal
conductivity. (The ductility will determine the amount of surface area of
the mesh in contact with the heat source and heat sink, an important
factor in the thermal resistance of the mesh.) It should also be noted
that the thinner the mesh and the denser the wire pattern, the lower the
thermal resistance. A 180 mesh screen with 0.0023-inch diameter wires has
been successfully employed by the present inventor.
In using the present invention, the flatness of the substrate or other
component to which the interface is coupled is an important consideration;
ground and lapped surface finishes will provide the best results.
Although a preferred application of the present invention is as an
interface between an IC die and a heat sink, the true scope of the
invention as described in the following claims is not limited to this
particular application. For example, the present invention may be used in
any assembly where there is a need for a material that is both
electrically and thermally conductive. In addition, the present invention
is not limited to the materials specified above (silicone grease,
paraffin, silver, copper, gold, etc.), since these materials may be
replaced with materials that function equivalently in the context of the
invention.
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